ESR Study of Ferroelastic Phase Transition in KH₃(SeO₃)₂ doped with Cr³⁺

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ESR spectra of Cr³⁺ at K-site in KH₃(SeO₃)₂ crystal have been studied. Below T_c, there are two kinds of inequivalent Cr³⁺ sites whose principal coordinate systems (X, Y and Z) of D-tensor are slightly rotated around Z-axis each other. The critical exponent value β was estimated to be 0.50±0.02 from the difference in principal value between the two inequivalent sites. The experimental results indicate that reorientation of SeO₃²⁻ groups is accompanied by proton ordering and K ion displacement in the ac plane perpendicular to the O(2)–H(2)–O(2) bond direction below T_c. ESR spectra corresponding to single domain were observed by application of shear stress X₄.